Gabriele Guidi received his M.S. degree in Electronic Engineering in 1988 from the University of Florence, and Ph.D. in 1992 from the University of Bologna, Italy. He joined the University of Florence as researcher in 1995 and moved to the Polytechnic of Milan in 2004 where he is now Associate Professor. He worked for more than 10 years on researches about ultrasound equipment for biomedical imaging. Since 2000 he reoriented his activity by applying imaging technologies to the field of non contact 3D measurement. He has been working on applications of three-dimensional acquisition and modelling in various fields, with special emphasis to Cultural Heritage documentation, Design and industrial applications. Dr. Guidi is Senior Member of the IEEE. He a component of the editorial board of the International Journal "Digital Applications in Archaeology and Cultural Heritage" (DAACH), published by Elsevier. He also works as reviewer of a few international journals such as “The IEEE Transactions on System, Man and Cybernetics”, “The IEEE Transactions on Image Processing” and “Machine Vision and Applications” (Springer). In 2012 Gabriele Guidi chaired the 18th International Conference on Virtual Systems and Multimedia held in Milan in (VSMM2012). In the previous years he has also been involved in the scientific committees of several conferences including “3DIM” (IEEE), “Videometrics” (SPIE), 3DArch (ISPRS) and Computer Applications and Quantitative Methods in Archaeology Conference (CAA).

Publications (11)

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Manufacturers often express the performance of a 3D imaging device in various non-uniform ways for the lack of
internationally recognized standard requirements for metrological parameters able to identify the capability of capturing
a real scene. For this reason several national and international organizations in the last ten years have been developing
protocols for verifying such performance.
Ranging from VDI/VDE 2634, published by the Association of German Engineers and oriented to the world of
mechanical 3D measurements (triangulation-based devices), to the ASTM technical committee E57, working also on
laser systems based on direct range detection (TOF, Phase Shift, FM-CW, flash LADAR), this paper shows the state of
the art about the characterization of active range devices, with special emphasis on measurement uncertainty, accuracy
and resolution.
Most of these protocols are based on special objects whose shape and size are certified with a known level of accuracy.
By capturing the 3D shape of such objects with a range device, a comparison between the measured points and the
theoretical shape they should represent is possible. The actual deviations can be directly analyzed or some derived
parameters can be obtained (e.g. angles between planes, distances between barycenters of spheres rigidly connected,
frequency domain parameters, etc.).
This paper shows theoretical aspects and experimental results of some novel characterization methods applied to
different categories of active 3D imaging devices based on both principles of triangulation and direct range detection.

The core of the paper is focused on the experimental characterization of four different 3D laser scanners based on Time
of Flight principle, through the extraction of resolution, accuracy and uncertainty parameters from specifically designed
3D test objects. The testing process leads to four results: z-uncertainty, xy-resolution z-resolution and z-accuracy. The
first is obtained by the evaluation of random residuals from the 3D capture of a planar target, the second from the
scanner response to an abrupt z-jump, and the last two from direct evaluation of the image extracted by different
geometric features progressively closer each other. The aim of this research is to suggest a low cost characterization
process, mainly based on calibrated test object easy to duplicate, that allow an objective and reliable comparison between
3D TOF scanner performances.

While 3D imaging systems are widely available and used, clear statements about the possible influence of material
properties over the acquired geometrical data are still rather few. In particular a material very often used in Cultural
Heritage is marble, known to give geometrical errors with range sensor technologies and whose entity reported in the
literature seems to vary considerably in the different works. In this article a deep investigation with different types of
active range sensors used on four types of marble surfaces, has been performed. Two triangulation-based active sensors
employing laser stripe and white light pattern projection respectively, and one PW-TOF laser scanner have been used in
the experimentation. The analysis gave rather different results for the two categories of instruments. A negligible light
penetration came out from the triangulation-based equipment (below 50 microns with the laser stripe and even less with
the pattern projection device), while with the TOF system this came out to be two orders of magnitude larger,
quantitatively evidencing a source of systematic errors that any surveyor engaged in 3D scanning of Cultural Heritage
sites and objects should take into account and correct.

Resolution analysis represents a 2D imaging topic for the use of particular targets for equipment characterization. These
concepts can be extended in 3D imaging through the use of specific tridimensional target object. The core of this paper is
focused on experimental characterization of seven different 3D laser scanner through the extraction of resolution,
accuracy and uncertainly parameters from 3D target object. The process of every single range map defined by the same
resolution leads to different results as z-resolution, optical resolution, linear and angular accuracy. The aim of this
research is to suggest a characterization process mainly based on resolution and accuracy parameters that allow a reliable
comparison between 3D scanner performances.

When TOF laser scanners were introduced on the market, their performances were rather poor, having in general a
measurement uncertainty in the range of centimeters. For this reason it was clear that their application was definitely
limited to environment and architecture survey, where the large size of the involved objects makes acceptable the relative
measurement error. But with the progressive improvement of technology, and the consequent increase in the
measurement precision, the potential range of purposes have been widened. In this paper an application to museum
objects have been considered. Studying the scanner performance when working at a low range, and using such results to
properly interpret the acquired data, it was possible to survey a famous wooden model of S. Peter basilica in Rome, remodeling
its shape with a 3D surface modeler. Resolution, precision and accuracy have been studied at distances ranging
from 1 to 3 meters, in working conditions similar to those imposed by the museum constrains. The results were used to
properly set-up some post processing steps instrumental characterization as a key step in the 3D modeling process, both
for increasing the geometrical data reliability, and for processing them (e.g. in the smoothing phase) in a way compliant
with their metrological characteristics.

The calibration of a range camera greatly influences the whole 3D acquisition and modeling process, allowing to
minimize the equipment inaccuracy. However, depending on the range camera "openness", we might have systems precalibrated
only once by the industrial manufacturer or systems requiring a regular (and mandatory) end-user calibration
before any scan session. Independently of the calibration approach, the metrological system characterization represents a
point of paramount importance for making the user aware of the actual performances of his equipment. This permits the
choice of appropriate resolution in 3D scan planning and allows to properly interpret the feedback indices during the
alignment of several range maps trough Iterative Closest Point (ICP). Finally, in polygonal model editing, the
modification of geometrical features is greatly helped by the awareness about the 3D capturing device performances.
These remarks are effective for both triangulation based instruments, like Minolta Vivid 910, ShapeGrabber SG100 and
SG1000 evaluated in this paper, or TOF based instruments. The proposed experimental method is based on post
processing of the range data produced by acquiring the surface of a precise test object with a 3D laser scanner. In this
procedure resolution, accuracy, and precision parameters are obtained sequentially, through the application of a set of
simple geometric processing steps. Such operating easiness make this approach a possible candidate as a mandatory step
in any 3D acquisition and modeling project.

This paper shows the applicability of non-contact 3D imaging technology to the dimensional monitoring of wooden
artworks. The results of a study on a wooden test artifact submitted to sudden environmental parameters changes
(temperature and humidity) are presented. It was possible to verify that the range maps generated by a 3D camera based
on optical triangulation have the necessary resolution to show dimensional variations in the order of a few tenths of a
millimeter. 3D models generated before and after the parameter variation were processed with a specific software in
order to highlight the amount of deformation through a color coded image. The results demonstrate that such technique
represents a unique instrument to capture and track the deformations of wooden artifacts before they become permanent.
This study has been conducted in collaboration with the laboratories for restoration of the "Opificio delle Pietre Dure" at
Florence, Italy.

In a 3D acquisition project range maps collected around the object to be modeled, need to be integrated. With portable range cameras these range maps are taken from unknown positions and their coordinate systems are local to the sensor. The problem of unifying all the measurements in a single reference system is solved by taking contiguous range maps with a suitable overlap level; taking one map as reference and doing a rototranslation of the adjacent ones by using an "Iterative Closest Point" (ICP) method. Depending on the 3D features over the acquired surface and on the amount of overlapping, the ICP algorithm convergence can be more or less satisfactory. Anyway it always has a random component depending on measurement uncertainty. Therefore, although each individual scan has a very good accuracy, the error's propagation may produce deviations in the aligned set respect to real surface points. In this paper a systematic study of the different alignment modality and the consequent total metric distortions on the final model, is shown. In order to experiment these techniques a case-study of industrial interest was chosen: the 3D modeling of a boat's hull mold. The experiments involved a triangulation based laser scanner integrated with a digital photogrammetry system. In order to check different alignment procedures, a Laser Radar capable to scan all the object surface with a single highly accurate scan, was used to create a "gold-standard" data set. All the experiments were compared with this reference and from the comparison several interesting methodological conclusions have been obtained.

Computer modeling through digital range images has been used for many applications, including 3D modeling of objects belonging to our cultural heritage. The scales involved range from small objects (e.g. pottery), to middle-sized works of art (statues, architectural decorations), up to very large structures (architectural and archaeological monuments). For any of these applications, suitable sensors and methodologies have been explored by different authors. The object to be modeled within this project is the "Plastico di Roma antica," a large plaster-of-Paris model of imperial Rome (16x17 meters) created in the last century. Its overall size therefore demands an acquisition approach typical of large structures, but it also is characterized extremely tiny details typical of small objects (houses are a few centimeters high; their doors, windows, etc. are smaller than 1 centimeter). This paper gives an account of the procedures followed for solving this "contradiction" and describes how a huge 3D model was acquired and generated by using a special
metrology Laser Radar. The procedures for reorienting in a single reference system the huge point clouds obtained after each acquisition phase, thanks to the measurement of fixed redundant references, are described. The data set was split in smaller sub-areas 2 x 2 meters each for purposes of mesh editing. This subdivision was necessary owing to the huge number of points in each individual scan (50-60 millions). The final merge of the edited parts made it possible to create a single mesh. All these processes were made with software specifically designed for this project since no commercial package could be found that was suitable for managing such a large number of points. Preliminary models are presented. Finally, the significance of the project is discussed in terms of the overall project known as "Rome Reborn," of which the present acquisition is an important component.

The French-Italian interferometric gravitational wave detector VIRGO is currently being commissioned. Its principal instrument is a Michelson interferometer with 3 km long optical cavities in the arms and a power-recycling mirror. This paper gives an overview of the present status of the system. We report on the presently attained sensitivity and the system’s performance during the recent commissioning runs.

The goal of the VIRGO program is to build a giant Michelson type interferometer (3 kilometer long arms) to detect gravitational waves. Large optical components (350 mm in diameter), having extremely low loss at 1064 nm, are needed. Today, the Ion beam Sputtering is the only deposition technique able to produce optical components with such performances.
Consequently, a large ion beam sputtering deposition system was built to coat large optics up to 700 mm in diameter. The performances of this coater are described in term of layer uniformity on large scale and optical losses (absorption and scattering characterization).
The VIRGO interferometer needs six main mirrors. The first set was ready in June 2002 and its installation is in progress on the VIRGO site (Italy). The optical performances of this first set are discussed. The requirements at 1064 nm are all satisfied. Indeed, the absorption level is close to 1 ppm (part per million), the scattering is lower than 5 ppm and the R.M.S. wavefront of these optics is lower than 8 nm on 150 mm in diameter. Finally, some solutions are proposed to further improve these performance, especially the absorption level (lower than 0.1 ppm) and the mechanical quality factor Q of the mirrors (thermal noise reduction).

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